1. Field of the Invention
The present invention relates to an image processing apparatus and an ultrasonic diagnosis apparatus, and particularly to an ultrasonic diagnosis apparatus wherein the movements of characterizing points (tags) obtained from an image of an organism are tracked, and information with regard to various local functions of the tissue are estimated and output based upon the above-described tracking of the movements of tags, thereby enabling useful clinical information to be provided.
2. Description of the Related Art
Quantitative evaluation of local movement of the heart or the like (contraction/expansion functions) is a matter of great importance for understanding the function thereof. It is well known that, in the case of ischemic heart disease, for example, the change in regional wall movement occurs due to the shortage of blood supplied from coronary arteries.
Concerning quantitative evaluation methods for the regional wall movement, a great number of conventional methods have been proposed. Examples include “MRI tagging (magnetic marking) method” disclosed in Japanese Unexamined Patent Application Publication No. 7-184877, “two-dimensional movement vector detection by B-mode image”, “tissue Doppler method”, and the like.
The MRI tagging (magnetic marking) method is a method specific to MRI (magnetic resonance imaging), wherein magnetic marks (tags) from electromagnetic waves are placed onto an MRI image as a grid, and quantitative evaluation of the temporal change in the tags is performed, so that the movement or distortion of organic tissue is visualized. The MRI tagging method is a method wherein grid points, which are magnetic marks referred to as tags, are taken as sample points so as to detect movement and display a scene of distortion of the grid, and corresponds to an analytical method which is referred to the Lagrange method in physics (continuum mechanics). By use of the Lagrange method, temporal tracking the sample points enables contraction and expansion of cardiac muscle or the like to be directly calculated as a tensor property.
The “two-dimensional movement vector detection by B-mode image” includes conventional methods such as a method wherein movement vectors are estimated based upon the peak position of the two-dimensional cross-correlation coefficients, an optical flow method using the gradient of the image density, as a method for detecting the movement in the direction orthogonal to an ultrasonic beam. Information to be displayed includes movement vectors, tracks, cross-correlation value, and so forth.
The tissue Doppler method is a method wherein the movement of tissue is detected using the ultrasonic pulse Doppler or a color Doppler, and basically, only the component in the direction of the ultrasonic beam is detected. A method has been also proposed wherein two-dimensional movement components are obtained by making an assumption for the direction of the movement. The estimated and displayed information includes the difference of the velocities between two sample points, the distortion obtained by integrating the above difference, and so forth.
On the other hand, the twisting or distortion of cardiac muscle, which cannot be readily detected with conventional arrangements, can be analyzed using the MRI tagging method. However, there are problems that an MRI apparatus is an expensive apparatus, and also image acquisition using tagging cannot be performed in real time.
Accordingly, in general, the obtained MRI image is an image for a time period of a plurality of cardiac beats, and evaluation of the wall movement for each cardiac beat can not be made. In particular, it is well known that evaluation of the expansibility requires time-resolution with high precision, and consequently, sufficient analysis cannot be readily performed using the MRI due to the time-resolution of MRI (50 ms to 100 ms).
To the contrary, with the use of the two-dimensional movement vector detection by ultrasonic B-mode, while tracking can be performed on relatively large tissue with clear contours such as endocardium and annulus, or the like, or interference patterns due to random ultrasonic scatter, which are referred to as “speckle pattern”, the trackable characterizing points cannot be readily specified.
Therefore, upon the two-dimensional movement vector detection by ultrasonic B-mode, temporal tracking arbitrary grid points within cardiac muscle cannot be performed as it can upon the tagging method in MRI.
If tracking is attempted with the use of the two-dimensional movement vector detection by ultrasonic B-mode, the temporal change (trail of movement, etc.)of only the trackable characterizing points can be displayed. Furthermore, while various types of improved methods have been proposed with regard to tracking algorithm itself, such as a method wherein compound processing is added to simple cross-correlation calculation, the precision of the methods is poor, and consequently the methods are hardly practiced in the clinical field. The characterizing points which are suitable to tracking must be selected in the event of performing tracking with high precision.
On the other hand, in the event of employing the tissue Doppler method, there is a problem that the apparatus is expensive due to necessity of having a cross-correlation computation circuit for Doppler calculation.
Moreover, the phase change (the change in a distance within the half-wavelength) detected by the tissue Doppler method is smaller than the movement amount of local myocardial portions (around 1 to 10 mm), and accordingly, the displacement of the myocardial portion is obtained by time integrating the detected instantaneous phase (velocity) in order to get the information with regard to the macroscopic movement of the cardiac muscle.
Therefore, accumulated error margin arisen by integrating the velocity information, leads to a problem the same as with performing the LaGrange analysis wherein a mark (tag) is placed onto a certain point, and direct tracking of the movement thereof is required. In particular, provided that time and spatial resolution is insufficient, interpolation processing for data is necessary in order to calculate movement amount, and the precision thereof also influences the final measurement precision.
Besides, with conventional arrangements, in either method described above, tracking of arbitrary positions within cardiac muscle cannot be performed, but specifying the initial position or region to be tracked must be made by manual operations, which is troublesome. That is, provided that a pointer or the like is placed onto a certain point, the point can be tracked. However, in the event of placing the pointer onto a point at which there are no structures, tracking of the point cannot be made. Consequently, a user must change the point on which the pointer has been placed, and procedures for selecting a trackable point one by one point by manual operations are troublesome for users.
Moreover, with regard to analysis methods, there is the problem of angular dependence. For example, whether the movement is expansion or contraction might depend on the direction parallel or orthogonal to the fiber of the same portion of cardiac muscle, and accordingly, different results are obtained depending upon the analysis direction and an incorrect diagnosis might be made. In other words, with the tissue Doppler method, basically tissue is one-dimensionally analyzed, i.e., the moving velocity of tissue in the beam direction is measured, so one-dimensional information is obtained, and consequently, the movement in other directions must be assumed (estimated) from projection components in the orthogonal direction. Moreover, tracking cannot be readily made in the other directions.